Stable isotope signatures and distribution of deepwater sponges in Lake Baikal

To better understand the species composition and nutrition of deepwater sponges in Lake Baikal, we determined the depth, habitat, and stable isotopic signatures for deepwater sponges. All sponge specimens except one were collected at depths below the photic zone and in a variety of benthic habitats including methane seeps and hydrothermal vents. The freshwater sponge species of Lake Baikal generally occurred in aggregations (fields, carpets, and belts), and mainly on hard substrates (different types of bedrock and argillite, and bitumen). New maximum depth records were established for three species. The deepwater sponges clustered into two groups, based on their isotopic values, with members of one group assimilating organic carbon from photosynthetic sources and exhibiting δ13C values (−32 to −26.9‰) similar to those reported previously for littoral sponges. Members of the second group, distinguished by highly depleted δ13C values (−65 to −37.3‰), participate in a chemosynthetic food web. δ15N values of the deepwater sponges ranged widely (2.1–16.7‰) suggesting that they assimilate nitrogen from environmental sources and possibly from their microbial symbionts. Conservation of deepwater sponge diversity is therefore, likely dependent on the diversity of resources found in different habitats.